How much does a full aquarium weigh?
Fresh water weighs 8.32 lb per US gallon (0.997 kg per liter), so the quick answer is "a bit more than 8 pounds a gallon plus the tank." The quick answer is wrong in both directions at once: it multiplies by the gallons on the label instead of what the tank holds, and it forgets everything that isn't water. Here is a 75 — 48 × 18 × 21 in, glass, 2 in of gravel, 30 lb of rock, filled to the rim — component by component:
| Component | Pounds | Kilograms | Share |
|---|---|---|---|
| Water (66 gal after gravel and rock) | 553 | 251 | 71% |
| Glass (8 mm panes) | 103 | 47 | 13% |
| Gravel, 2 in | 95 | 43 | 12% |
| Rock | 30 | 14 | 4% |
| Tank and contents | 781 | 354 | 100% |
Computed by this page's engine. Glass thickness is the typical value for a 21 in tank; trim and center brace add a few pounds.
That's 781 lb before the stand — about 11.8 lb per gallon of actual water, which is where the hobby's "10 pounds a gallon" rule comes from. Add a 60 lb cabinet and 15 lb of filter and lights and the floor sees 856 lb (388 kg). Multiplying the label by 8.34 would have given you 624 — 27% light.
Aquarium weight chart: standard sizes, filled
Glass tanks at their standard outside dimensions, fresh water to the rim, two inches of gravel, no rock, no stand. "Empty" is the bare glass at the typical thickness for that height.
| Tank | Empty | Water | Gravel | Filled | lb/ft² |
|---|---|---|---|---|---|
| 5.5 gallon | 6 lb | 38 lb | 14 lb | 59 lb · 27 kg | 66 |
| 10 gallon | 10 lb | 74 lb | 22 lb | 106 lb · 48 kg | 77 |
| 15 gallon | 12 lb | 108 lb | 32 lb | 152 lb · 69 kg | 76 |
| 20 gallon long | 15 lb | 135 lb | 40 lb | 190 lb · 86 kg | 76 |
| 20 gallon high | 26 lb | 144 lb | 32 lb | 201 lb · 91 kg | 101 |
| 29 gallon | 40 lb | 203 lb | 39 lb | 282 lb · 128 kg | 113 |
| 37 gallon | 63 lb | 246 lb | 39 lb | 348 lb · 158 kg | 139 |
| 40 gallon breeder | 42 lb | 330 lb | 72 lb | 444 lb · 202 kg | 99 |
| 55 gallon | 91 lb | 411 lb | 68 lb | 570 lb · 258 kg | 131 |
| 65 gallon | 115 lb | 489 lb | 70 lb | 674 lb · 306 kg | 150 |
| 75 gallon | 103 lb | 577 lb | 95 lb | 776 lb · 352 kg | 129 |
| 90 gallon | 143 lb | 656 lb | 94 lb | 893 lb · 405 kg | 149 |
| 120 gallon | 164 lb | 884 lb | 127 lb | 1,175 lb · 533 kg | 147 |
| 125 gallon | 144 lb | 870 lb | 143 lb | 1,158 lb · 525 kg | 129 |
| 150 gallon | 270 lb | 1,149 lb | 140 lb | 1,560 lb · 707 kg | 173 |
| 180 gallon | 279 lb | 1,376 lb | 189 lb | 1,844 lb · 837 kg | 154 |
| 210 gallon | 389 lb | 1,584 lb | 187 lb | 2,160 lb · 980 kg | 180 |
Computed by this page's engine. Add your stand (60–90 lb for a wood cabinet at 4 ft), rock, and any sump. Saltwater adds 2.8% to the water line.
Will my floor hold my aquarium?
US residential floors are designed to a live load of 40 lb per square foot in living areas (30 in bedrooms) — and a 75 with gravel and rock sits at 130 lb/ft². Read those two numbers side by side and you'd never put a fish tank above a basement. Yet 75s live on second floors everywhere, and they're fine, because the 40 is not what it looks like.
The 40 psf is a uniform design load: every square foot of the room loaded at once, all the way across every joist, and the floor still has to stay within its deflection limit — with a safety factor on top. A 12 × 14 ft room is designed to carry 6,720 lb spread evenly. A tank is a concentrated load on a small patch: your 75 puts 856 lb across the two or three joists under it, and each of those joists is sized to carry its own strip of the room fully loaded. Concentrated loads still matter — they matter where they sit — but the psf comparison alone is the wrong test.
The practical rules, in order of how much they help:
1. Put it across the joists, not along one. A 4 ft tank set perpendicular to the joists spreads across three or four of them; set parallel, it can ride a single joist. Joists usually run the short way across a room and perpendicular to the ridge line. 2. Put it against a load-bearing wall, ideally an exterior one, where the joists are supported rather than mid-span. 3. Under about 550 lb (a 55 and everything below it) you can stop worrying on any sound floor. Between there and about 1,000 lb — the 75, 90 and 120 — the two placement rules above are the whole job. Above ~1,000 lb — 125s and up, and any big reef with a full sump — you're at the weight of a grand piano, and the honest answer is a look at the framing (joist size, span, condition, and what's below) or a ground floor on slab. Old houses, notched joists, long spans and mid-room placements move every threshold down.
| Load | About | Footprint | lb/ft² |
|---|---|---|---|
| Queen waterbed | 1,800 lb | 33 ft² | 55 |
| Refrigerator | 300 lb | 6 ft² | 50 |
| Upright piano | 500 lb | 5 ft² | 100 |
| Four adults standing close | 700 lb | 6 ft² | 117 |
| 75 gallon, gravel and rock | 781 lb | 6 ft² | 130 |
| 180 gallon, gravel | 1,844 lb | 12 ft² | 154 |
Waterbed, appliance and people figures are rough published values; the two tanks are this page's engine.
Glass vs acrylic: how much lighter is acrylic?
Acrylic is 2.1× less dense than glass but is used about 1.5× thicker for the same height, so an acrylic tank ends up around a third lighter — for the tank. On the 75 above that is 103 lb of glass versus 74 lb of acrylic: a saving of 30 lb on a 781 lb setup, roughly 4%. Acrylic earns its keep for moving and shipping and for very large tanks; it doesn't change the answer about your floor. The water does.
Saltwater, sand, aquasoil and rock — what changes the number
| Same 75, but… | Tank and contents | vs. baseline |
|---|---|---|
| Bare glass and fresh water, no gravel or rock | 716 lb | −65 lb |
| Baseline: 2 in gravel + 30 lb rock | 781 lb | — |
| Reef salinity instead of fresh | 797 lb | +16 lb |
| 2 in sand instead of gravel (no rock) | 773 lb | +57 lb over bare, net of displaced water |
| 2 in aquasoil instead of gravel (no rock) | 731 lb | +15 lb over bare, net |
Computed by this page's engine. Substrate and rock each push out water they replace, so they add less than they weigh; rock at 10 lb per displaced gallon.
Salt water is only 2.8% heavier — 16 lb on a 75. Where a reef really adds weight is the sump: 20 gallons in the cabinet is another 171 lb on the same footprint, and 100+ lb of rock is normal. Enter both. On the freshwater side, an aquasoil bed is about half the weight of the same depth of gravel; a deep sand bed is about the same as gravel.
What the stand carries, and what the floor carries
The result panel splits them. On the stand is tank plus water, substrate and rock — what the stand's rating has to cover. Total on the floor adds the stand itself, the sump water and the equipment. One more number worth knowing: a four-legged metal stand puts a quarter of the total on each foot. For the 75 with a stand that's about 214 lb per leg on a patch the size of a coin, which is why metal stands dent soft floors and why a sheet of plywood under a stand isn't just for leveling. A cabinet stand spreads the same load along two long rails and is easier on both the floor and the tank's bottom pane.